The invention relates to a method for the denitrification of bypass exhaust gases in a plant for producing cement clinker, wherein the plant has a rotary kiln for the sintering of raw meal to cement clinker and has a calciner for the deacidification of the raw meal, downstream of the rotary kiln in the kiln exhaust gas flow direction, the rotary kiln has a rotary kiln inlet chamber which is connected directly or via a kiln riser duct to the calciner, and the bypass exhaust gas is drawn off in the region of the rotary kiln inlet chamber. The invention further relates to a corresponding plant for the denitrification of bypass exhaust gases in the production of cement clinker, comprising a rotary kiln for the sintering of raw meal to cement clinker, the rotary kiln having a rotary kiln inlet chamber, a calciner for the deacidification of the raw meal, the rotary kiln inlet chamber being connected directly or via a kiln riser duct to the calciner, and a takeoff device for drawing off the bypass exhaust gases from the region of the rotary kiln inlet chamber.
Within the overall operation of cement production, plants are employed in which silicate-containing and carbonate-containing raw meal is sintered to cement clinker in a rotary kiln. The sintering in the rotary kiln, which proceeds at temperatures of up to around 1450° C., produces flue gases which, as hot exhaust gases, leave the rotary kiln in the direction opposite to the flow of material, through the inlet chamber of the rotary kiln. In the normal instance, the kiln exhaust gases then flow into a calcining zone, in which the raw meal is deacidified. The calcining zone is most often formed in a kiln riser duct or in a calciner, or in a kiln riser duct and a calciner downstream (in the gas flow direction). The flue gas then flows further into a heat exchanger, designed, for example, as a multistage cyclone heat exchanger, which serves for the preheating of the raw meal. A problem affecting the operation of cement clinker production is the formation and/or release of a series of pollutants. In particular, on account of the high temperatures of the burner flames (about 1800° C. to 2000° C.), nitrogen oxides (NOx) are formed in the rotary kiln by combustion of the nitrogen which is contained within the atmospheric air. The fuel required as well, especially when using secondary fuels such as the replacement fuels obtained from waste, is another source of nitrogen oxides. Given that nitrogen oxides have adverse consequences for people and the environment—as a cause of acid rain, and through breakdown of ozone in the stratosphere, for example—there are strict limits on the emission of nitrogen oxides into the atmospheric environment. In the course of cement production, therefore, methods must be employed for the denitrification of the flue gases.
A further problem is that the raw materials for cement clinker production, and also the fuels employed, especially secondary fuels, contain by-constituents (alkali metal compounds, chlorine, sulfur compounds, heavy metals, etc.) which not only may be detrimental to the quality of the combustion process and/or of the cement clinker, but may also form deleterious substance circuits within the plant for cement clinker production. In the rotary kiln, for example, there is evaporation of alkali metal sulfates and alkali metal chloride compounds, e.g., potassium chloride (KCl). With the kiln exhaust gas, these compounds pass through the kiln inlet chamber into the calcining zone and the heat exchanger, and they condense on the raw meal particles in the cooler regions, and pass with the material stream back into the rotary kiln, where they evaporate again. Further to the disadvantages of such substance circuits for the cement clinker and the combustion process, rapid cooling and condensation of these compounds give rise, through solidification, to caking on the walls of the cooler sections of the circuit, which may cause the plant to become blocked over time.
For the purpose of suppressing substance circuits of this kind in plants for cement clinker production, and for reducing the level of circuit-forming substances, the patent specification DE 197 18 259 B4 discloses drawing off, as a bypass, a part of the flue gas that flows as kiln exhaust gas from the rotary kiln, in the region of the rotary kiln inlet chamber. The phrase “in the region of the rotary kiln inlet chamber,” here and below, refers consistently to removal from the rotary kiln inlet chamber or else to removal from the lower end of any kiln riser duct there may be. Even the bypass exhaust gas, however, contains a higher level of nitrogen oxides, and so flue gas denitrification must be performed for the bypass exhaust gas as well.
One widespread method for the denitrification of flue gases involves feeding the NOx-affected flue gases with an aqueous ammonia solution, ammonia (NH3) or ammonia-releasing compounds in a reaction space (see, for instance, the proposal contained in EP 0 854 339 A1). Denitrification then proceeds by the process of selective non-catalytic reduction (SNCR), in which ammonia is converted by thermolysis with the nitrogen oxides into nitrogen and water. These reactions proceed preferably in a temperature window from 800° C. to more than 950° C. For effective implementation, furthermore, it is necessary to realize a timespan which requires precise establishment, and at any rate a minimum time, for the processes within the reaction space. In the case of the desired denitrification of bypass exhaust gas, however, denitrification by the SNCR process proves to be problematic, since the temperatures of the bypass exhaust gas drawn off are too high and, in addition, compliance with the residence time in the reaction space imposes exacting requirements on the operating regime. It is true that the temperature of the exhaust gases in the kiln falls from up to about 1250° C. on entry into the rotary kiln inlet chamber and the lower part of any kiln riser duct that may be present, and yet the gas temperatures of around 1150° C. which still prevail at this point are still so high that reducing agents added would undergo combustion.
One known procedure (DE 197 18 259 B4, DE 199 10 927 A1) is to carry out rapid cooling, preferably to just a few hundred ° C., of the hot bypass exhaust gas stream in mixing chambers, in which a cooling medium such as water or air is injected and is mixed as extensively as possible with the gas stream. This does have the advantage that evaporated substances which are drawn off from the pollutant circuits condense on the surfaces of the particulate solids and can then be removed together with these solids by means of dust filters. For effective denitrification, however, this operation is not suitable.
The German patent application with the number 10 2013 016 701.9 discloses a method for the denitrification of bypass exhaust gases in a plant for producing cement clinker by initially cooling the bypass exhaust gas to temperatures between 260° C. and 400° C. in a mixing chamber, for instance. This is followed by the feeding of the cooled bypass exhaust gas with substances containing ammonia, containing urea and/or containing ammonium. A consequence of this is that the nitrogen oxides are subject to selective chemical reduction over a catalyst which is present in a ceramic filter arrangement and/or which immediately follows the ceramic filter arrangement, in the presence of the substances containing ammonia, urea and/or ammonium. In terms of method, therefore, the denitrification in this case is based on the process of selective catalytic reduction (SCR). Capital costs and operating costs for the catalyst and/or catalytic filter required in the case of SCR, however, are comparatively high. Especially when volume flows of bypass exhaust gas are comparatively low, this process may prove economically to be not very advantageous, thus illustrating the advantageous nature of alternative procedures.